Hypertriglyceridemia is emerging as important predictor of atherosclerosis, and recent evidence suggests related phenotypes of triglycerides (TGs), such as TG remnant particles and small LDL particles, are particularly atherogenic. There is considerable variation in the response of TGs and related phenotypes to the environment. The aim of the proposed study is to characterize the genetic basis of the variable response of TGs to two environmental contexts, one that raises TGs (dietary fat), and one that lowers TGs (fenofibrate treatment). We will recruit 2,400 family members from 3-generational pedigrees of the ongoing NHLBI Family Heart Study (FHS) in two genetically homogeneous centers (Minneapolis and Salt Lake City). We will collect measurements before and after a dietary fat challenge to assess postprandial TGs and related atherogenic phenotypes (VLDL TGs, chylomicron TGs, TG remnant particles, HDL and LDL particle sizes, total cholesterol, LDL-C, and HDL-C). In families with 2 or more members in a sibship with TGs >= 130 mg/dl, we will conduct a short-term, placebo-controlled, randomized trial of fenofibrate in all willing and eligible family members (anticipated sample size = 1,200). A two-period crossover design will be executed with a 2-week washout between two 3-week treatment periods (placebo or micronized fenofibrate, 160 mg). About 1,000 family members have a Marshfield genome marker set available as part of NHLBI FHS; the remaining 1,400 will be typed using the same marker set. We will conduct genome-wide linkage analyses using state-of-the-art methods to localize novel genetic loci contributing to TG response in the context of fat loading and fenofibrate treatment. We will type 15 single nucleotide polymorphisms (SNPs) in ten candidate genes known to contribute to the response of TGs to dietary fat and fenofibrate, and create haplotypes for association studies. We will use combinatorial partitioning methods and neural networks to test association of the individual SNPs and haplotypes with response to the two environmental interventions. The identification of genetic loci that predict TG response in the presence of two disparate contexts, fat loading and fibrate therapy, may provide insights into genetic pathways (a) predisposing to hypertriglyceridemia, ultimately leading to avenues for primary prevention, and (b) predicting response to TG lowering, leading to new drug targets for hypertriglyceridemia.